Composite Materials Research Progress

276
S.C. Tjong
developed in the past two decades for various automobile, aerospace, electronic packaging
and other structural applications. Many factors affect the mechanical properties of DRA
composites including matrix alloy composition, reinforcement material, reinforcement size,
shape, volume fraction and distribution, nature of the matrix-reinforcement interface, etc. The
reinforcement materials generally should possess significantly higher specific and specific
strength, as well as high melting temperature compared to the matrix alloy. Ceramic
reinforcement has the advantage of a relatively low density and high elastic modulus. Typical
ceramic particles commonly used to reinforce aluminum and its alloys including SiC, B4C,
Si3N4, AlN, Al2O3, TiC, TiB2, etc Particle reinforced composites are conventionally prepared
either via powder metallurgy (PM) or liquid metallurgy, in which the reinforcing particles
with sizes of several microns are directly incorporated into solid or liquid aluminum,
respectively. The composites thus prepared can be viewed as ex-situ MMCs. However,
ceramic microparticles fracture readily during mechanical loading, leading to low toughness
of the composites [1-3]. Figs. 1(a)-1(b) show typical fracture morphology of ceramic
microparticles in Al-based composites during tensile loading. Furthermore, reinforcement
material such as SiC is not thermodynamically stable and thus can react with aluminum
matrix during the composite fabrication and service at elevated temperatures. Efforts have
been made to overcome the occurrence of such difficulties by developing novel in-situ
processing. In the process, the reinforcing particles are directly formed in a metallic matrix by
chemical reactions between constituent elements during the composite fabrication [4, 5].
Accordingly, very fine in-situ particles with diameters down to submicrometer scale ( > 100
nm) can be synthesized and dispersed more uniformly within aluminum matrix [6]. The
formation of clean, ultrafine and thermally stable ceramic reinforcements rendering the in-situ
composites exhibit excellent mechanical properties.
Figure 1. SEM fractographs showing fracture and decohesion of alumina particles of (a) 6061
Al/20vol.%Al2O 3 and (b) 7005 Al /10vol.% Al 2O 3 composites tensile tested at room temperature [3].
The successful synthesis of large-scale ceramic, metallic and intermetallic nanoparticles
in recent years has motivated materials scientists to develop novel metal-matrix
nanocomposites with excellent mechanical properties for advanced structural engineering